This invention relates to a building system in which an upper structure is relatively rigidly connected to a base structure during normal conditions but which, when subjected to seismic disturbances, results in a relatively flexible interconnection between the upper structure and the base structure. The system disclosed herein is particularly useful in connection with offshore structures in which a large portion of the upper structure is surrounded by water to increase significantly its effective mass in the event of earthquake. Such offshore structures include major bridge piers, nuclear generator facilities, platforms for storing liquified gas and petroleum products, oil drilling platforms and oil production platforms.
Building systems have previously been devised for connecting an upper structure to a base structure relatively rigidly under normal conditions and relatively flexibly under earthquake conditions. In one such system, upper structures have been supported an elastomer pads formed of interleaved layers of steel and elastomers bonded together to permit lateral shifting of the structre under earthquake conditions. In such systems, the rigid connection under normal conditions is provided by horizontal restraining bars which connect the upper structure to the base structure in a manner which causes the bars to fail by torsion in the event of an earthquake, thereby enabling the elastomer pads to permit lateral shifting of the upper structure. In another system, lateral restraint has been provided by horizontally oriented pots filled with lead which is extruded from the pots under earthquake conditions to provide a less rigid lateral support than under normal conditions.
In earlier application Ser. No. 644,017 filed Dec. 24, 1975, a system was disclosed which included laterally oriented rods which rigidified a normal connection but failed under earthquake conditions, and an elastomeric material which provided a more flexible connection after failure of the rods.
According to the present invention, the connection between the base structure and the upper structure is provided by connector means, some of which are relatively rigid but fail by buckling under earthquake conditions, and others of which are flexible so as to flex without failing when the system is subjected to seismic disturbances. The flexible members may be stressed load-bearing elements, and the rigid members may be unloaded and arranged only to sustain forces produced by lateral relative movement between the structures. Preferably, the buckling rigid connectors are vertically oriented fluted tubes and the flexible connectors are vertically oriented cylindrical tubes. Both types of tubes are symmetrical with respect to their individual vertical axes so as to provide omnidirectional flexion and resistance to lateral shifting between the upper structure and the base structure. In event of buckling of the rigid members, a standby replacement set of such members may be released for movement to operable positions.